If you're not happy with the way Firefox uses space on your widescreen monitor, developer Johnathan Weare has a nice guide for setting up Firefox in widescreen mode, which he's dubbed Widefox.

Johnathan's custom layout moves the tabs to the right side of your browser (which should be a familiar home for anyone who likes moving their Windows sidebar or Mac dock to the side of their screen). If you like to maximize your browser and end up with a lot of unused whitespace on the sides, the Widefox set up is a nice idea. — Adam Pash